1. Field of the Invention
The present invention relates to directivity switchability in an antenna having wideband characteristics suitable for the transmission or reception of a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range.
2. Description of the Related Art
For two reasons, wireless devices are desired which are capable of operating in a much wider band than conventionally. A first reason is the need for supporting short-range wireless communication systems, for which the authorities have given permission to use a wide frequency band. A second reason is the need for a single terminal device that is capable of supporting a plurality of communication systems which use different frequencies.
For example, a frequency band from 3.1 GHz to 10.6 GHz, which has been allocated by the authorities to short-range fast communication systems, corresponds to a bandwidth ratio as wide as 109.5%. As used herein, “a bandwidth ratio” is a bandwidth, normalized by the center frequency f0, of a band. Patch antennas have bandwidth ratio characteristics of less than 5%, and ½ wavelength slot antennas have bandwidth ratio characteristics of less than 10% (both known as basic antenna structures), but with such bandwidth ratio characteristics, it is very difficult cover the entirety of the aforementioned band. To take for example the frequency bands which are currently used for wireless communications around the world, a bandwidth ratio of about 30% is required in order to cover from the 1.8 GHz band to the 2.4 GHz band with the same antenna. In order to simultaneously cover from the 800 MHz band to the 2.4 GHz band, a bandwidth ratio of 100% or more is required. Thus, as the number of systems to be supported by the same terminal device increases, and as the frequency band to be covered becomes wider, the need will increase for a wideband antenna, this being a solution for realizing a simple terminal device structure. Moreover, since a stronger need to suppress reflected interference waves has emerged due to signals becoming faster, it is strongly desired to realize an antenna which has not only wideband characteristics but also directivity switching properties while having a small shape. In the case of a wireless system in which wideband signals are globally used, it is necessary to realize an antenna which satisfies all of: wideband characteristics; directivity switching properties; and maintenance of the main beam direction within a wide operating band, while having a small shape.
The ¼ wavelength slot antenna, shown in schematic diagrams in FIGS. 25A to 25C, is one of the most basic planar antenna structures, and is known to attain a bandwidth ratio value of about 15%. FIG. 25A is an upper schematic see-through view; FIG. 25B is a schematic cross-sectional view taken along line AB; and FIG. 25C is a schematic see-through rear view, as seen through the upper face side.
As is shown in these figures, a feed line 115 exists on the upper face of a dielectric substrate 103. A recess is formed in the depth direction from an edge 105 of a finite ground conductor 101, which in itself is provided on the rear face. Thus, the recess functions as a slot 109 having an open end 111. The slot 109 is a circuit which is obtained by removing the conductor completely across the thickness direction in a partial region of the ground conductor 101, and exhibits a lowest-order resonance phenomenon near a frequency such that its slot length Ls corresponds to a ¼ effective wavelength. The feed line 115, which partly opposes and intersects the slot 109, excites the slot 109. The feed line 115 is connected to an external circuit via an input terminal 201. Note that, in order to establish input matching, a distance t3 from an open end point 125 of the feed line 115 to the slot 109 is typically set to a length of about a ¼ effective wavelength at the center frequency f0.
Japanese Laid-Open Patent Publication No. 2004-336328 (hereinafter “Patent Document 1”) discloses a structure for operating a ¼ wavelength slot antenna at a plurality of resonant frequencies. FIG. 26A shows a schematic structural diagram thereof. A ¼ wavelength slot 109, which recesses into a partial region of a ground conductor 101 on the rear face of the dielectric substrate 103, is excited at a feeding site 113, whereby a usual antenna operation occurs. Usually, the resonant frequency of a slot antenna is defined by a loop length of the slot 109. However, a capacitor element 16 which is provided between a point 16a and a point 16b according to Patent Document 1 is prescribed so as to allow a signal of any frequency that is higher than the intended resonant frequency of the slot 109 to pass through, thus making it possible to vary the resonator length Ls of the slot based on frequency. Specifically, at lower frequencies, as shown in FIG. 26B, the resonator length of the slot does not change from its usual value, and therefore is determined by the physical length of the recess structure. At higher frequencies, as shown in FIG. 26C, the antenna operates so that the slot has a resonator length Ls2 which is shorter than its physical resonator length Ls in high-frequency terms. Thus, Patent Document 1 describes that a single slot resonator structure can attain a multiple resonance operation.
Non-Patent Document 1 (“A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros” IEEE Antennas and Wireless Propagation Letters, vol. 2, 2003, pages 194 to 196) discloses a method for realizing a wideband operation of a ½ wavelength slot antenna. As mentioned above, one input matching method for the slot antenna shown in FIG. 25 has conventionally been to excite the slot resonator 109 at a point where a ¼ effective wavelength at the center frequency f0 is obtained, beginning from the open end point 125 of the feed line 115.
However, in Non-Patent Document 1, as shown in FIG. 27 (which shows an upper schematic see-through view), the line width of a feed line 115 is reduced in a region spanning a distance corresponding to a ¼ effective wavelength at f0, from an open end point 125 of the feed line 115 toward an input terminal 201, thus forming a resonator. The resultant inductive resonator region 127 is coupled to a slot 109 in an approximate center thereof.
Non-Patent Document 1 describes that the introduction of the inductive resonator region 127 increases the number of resonators operating near the operating band into two within the circuitry, these resonators being strongly coupled to each other, so that a multiple resonance operation is obtained. FIG. 2(b) of Non-Patent Document 1 corresponds to a frequency dependence of return intensity characteristics in the case where: a substrate having a dielectric constant 2.94 and a height of 0.75 mm is used; a slot length (Ls) of 24 mm and a design frequency of 5 GHz are assumed; a ¼ wavelength line in the inductive resonator region of the feed line 115 has a line-length (t1+t2+Ws) of 9.8 mm, with a line width W2 of 0.5 mm; and the offset distance (Lo) between the feed line 115 and the slot center is varied from 9.8 mm to 10.2 mm. Under any of these offset distance conditions, return intensity characteristics as good as −10 dB or less are obtained with a bandwidth ratio 32% (from near 4.1 GHz to near 5.7 GHz). As shown in comparison with respect to the measured characteristics in FIG. 4 of Non-Patent Document 1, such band characteristics are much better than the bandwidth ratio of 9% of a usual slot antenna which is supposedly produced under the same substrate conditions.
On the other hand, various techniques have been proposed over the years for changing the directivity of an antenna and subjecting an emitted beam for scanning. For example, some methods, e.g., adaptive arrays, allow a signal which is received via a plurality of antennas to be processed in a digital signal section to equivalently realize a beam scanning. Other methods, e.g., sector antennas, place a plurality of antennas in different orientations in advance, and switch the main beam direction through switching of a path on the feed line side. There are also methods which place reflectors and directors (which are unfed elements) near an antenna to tilt the main beam direction.
Japanese National Phase PCT Laid-Open Publication No. 2003-527018 (hereinafter “Patent Document 2”) discloses, as a sector antenna utilizing a slot antenna, a sector antenna structure in which a plurality of slot antennas are radially placed to realize switching of the main beam direction through switching of a path on the feed line side. In Patent Document 2, a Vivaldi antenna which is known to have ultrawideband antenna characteristics is used as an antenna to realize global switching of the main beam direction of emitted electromagnetic waves having ultrawideband frequency components.
Moreover, Japanese Laid-Open Patent Publication No. 2005-210520 (hereinafter “Patent Document 3”) discloses an example of a variable antenna which employs unfed parasitic elements for tilting a main beam direction in which emission from a radiation slot element occurs. In the variable antenna shown in FIG. 28, in proximity, a ½ effective wavelength slot resonator which is excited by a feed line 115 as a radiator (slot) 109 and unfed slot resonators serving as parasitic elements 109x and 109y are placed on a ground conductor 101. Through adjustment of the slot lengths of the parasitic elements 109x and 109y, switching can be made as to whether the parasitic elements function as directors or reflectors relative to a reflector, thus varying the direction of an emitted beam from the radiator. In order to allow the parasitic elements 109x and 109y to function as directors, the slot lengths of the parasitic elements may be adjusted to be shorter than the slot length of the radiator. In order to allow the parasitic elements 109x and 109y to function as reflectors, the slot lengths of the parasitic elements may be adjusted to be longer than the slot length of the radiator. In order to adjust a slot length, a slot length which is longer than necessary is prescribed on the circuit board; and, in a state of allowing the element to function as a slot circuit with a short slot length, somewhere along the slot length, selectively conduction is achieved by means of a switching element 205a or 205b so as to astride the slot along the width direction between portions of ground conductor. Patent Document 3 mentions use of MEMS switches as an exemplary method of implementing the switching elements 205a and 205b. 
In conventional slot antennas, it has been impossible, with a small structure, to simultaneously satisfy all of: widebandness; maintenance of the main beam direction within the operating band; and a function of globally switching the main beam direction in a drastic manner.
Firstly, the operating band of a usual slot antenna, which only has a single resonator structure within its structure, is restricted by the band of its resonance phenomenon. As a result of this, the frequency band in which good return intensity characteristics can be obtained only amounts to a bandwidth ratio of about 10% to 15%.
On the other hand, although the antenna of Patent Document 1 realizes a wideband operation because of a capacitive reactance element being introduced in the slot, it fails to disclose any function of drastically switching directivity. Moreover, it is well conceivable that an additional part such as a chip capacitor is required as the actual capacitive reactance element, and that variations in the characteristics of the newly-introduced additional part may cause the antenna characteristics to vary. Moreover, Patent Document 1 fails to disclose any directivity switching function of globally switching the main beam direction of an antenna with wideband characteristics.
Also in the example of Non-Patent Document 1, where a plurality of resonators are introduced in the structure in order to improve the band characteristics based on coupling between the resonators, the bandwidth ratio characteristics are only as good as about 35%, which needs further improvement. The upper schematic see-through view of FIG. 27 (which is modeled after FIG. 1 of Non-Patent Document 1) illustrates the slot width Ws to be of a small dimension. However, under the conditions for obtaining the aforementioned wideband characteristics, the slot width Ws will have to be set to 5 mm, which accounts for more than half of the length of ¼ wavelength region, i.e., 9.8 mm. When a desire for downsizing the antenna permits only a limited area for accommodating the slot, it may become necessary to fold up the linear-shaped slot, for example. Thus, a structure which requires a large Ws value in order to obtain wideband characteristics will be difficult to be downsized by nature. Furthermore, Non-Patent Document 1 fails to disclose any directivity switching function of globally switching the main beam direction of an antenna with wideband characteristics.
In the antenna disclosed in Patent Document 2, four slot antennas, most of whose constituent elements are not shared, are radially placed within the structure, and a driving method is used which switches the feed circuit for each slot antenna, whereby a function of switching the main beam direction is realized. However, the antenna structure is very large, thus presenting a problem in realizing a small-sized communication terminal.
In the antenna disclosed in Patent Document 3, too, slot antennas whose constituent elements are not shared are placed in parallel, thus presenting a problem from the standpoint of downsizing. Moreover, there is only a limited frequency band in which the slot antennas to be used as parasitic elements function as directors or reflectors, thus resulting in a problem in that the main beam direction of the antenna may possibly change to a different direction within the operating frequency band. Therefore, the antenna disclosed in Patent Document 3 fails to satisfy the requirement as to maintenance of the main beam direction within the band.